1. Field of the Invention
The subject invention generally relates to the isolation glass used in multitrack magnetic heads and, more particularly, to a new glass formulation with improved wear properties for use as the isolation glass.
2. Description of the Prior Art
Lead oxide based glasses are commonly used for the isolation glass in multitrack tape heads because of their compatibility with Nickel-Zinc (NiZn) ferrite. A typical example of a lead oxide based glass in common use for multitrack tape heads is IBM 400 glass. Table 1 details the composition IBM 400 glass.
TABLE 1 ______________________________________ CONSTITUENT WEIGHT PERCENT ______________________________________ Lead Oxide (PbO) 68.0 .+-. 0.5% Silicon Oxide (SiO.sub.2) 20.8 .+-. 0.5% Boron Oxide (B.sub.2 O.sub.3) 7.0 .+-. 1.5% Aluminum Oxide (Al.sub.2 O.sub.3) 4.2 .+-. 1.5% Remainder 0.5% Maximum ______________________________________
IBM 400 glass has an annealing point (glass viscosity=10.sup.13 poise) of 435.degree. C., a softening point (glass viscosity=10.sup.7.6 poise) of 530.degree. C., a working point (glass viscosity=10.sup.4 poise) of 695.degree. C. and a coefficient of thermal expansion equal to 70*10.sup.-7 .+-.3.0*10.sup.-7 /.degree. C. IBM 400 glass has superior properties in terms of its compatibility with NiZn ferrite; however, the glass has a high wear rate compared to that of the ferrite in the magnetic tape head.
Borosilicate based glasses have higher hardness and better wear attributes compared to lead oxide glasses. Borosilicate glass, specifically Corning.RTM. glass code 7740, has an annealing point of 565.degree. C., a softening point of 821.degree. C., a working point of 1252.degree. C., and a coefficient of thermal expansion equal to 32.5*10.sup.-7 /.degree. C. Lead borosilicate glass, specifically Corning.RTM. glass code 7570 has an annealing point of 363.degree. C., a softening point of 440.degree. C., a working point of 560.degree. C., and a coefficient of thermal expansion equal to 84*10.sup.-7 /.degree. C. However, neither borosilicate glass having Corning.RTM. glass code 7740 nor lead borosilicate glass having Corning.RTM. glass code 7570 alone are acceptable for bonding with NiZn ferrite.
U.S. Pat. No. 4,638,387 to Mukasa et al. discloses the use of borosilicate glass as an impregnating material for filling fine voids in a sliding contact member used in conjunction with a magnetic tape head. U.S. Pat. No. 4,069,360 to Yanagisawa et al. discloses the use of a polysilicate layer, which may be borosilicate glass, coated with a lubricant to protect and reduce frictional wearing of the recording disc on which it is coated and the surface of the magnetic recording head. U.S. Pat. No. 4,170,032 to Yokoyama et al. discloses a magnetic head wherein borosilicate glass is sputter deposited in the gap area. U.S. Pat. No. 4,110,804 to Castrodale et al. discloses the use of lead borosilicate glass for bonding parts of the transducer in a read/write and tunnel erase magnetic head assembly. None of the above-patents show or discuss the use of borosilicate based glass as a closure glass in a multitrack tape head.
Prior art multitrack heads suffer from excessive wear of the closure glass. When the closure glass wears down at a faster rate than the ferrite, the tape head becomes unreliable. The superior wear characteristics of borosilicate based glasses have not been applied in multitrack heads because of their incompatibility with NiZn ferrite.